Researchers at the Energy Storage Research Department at the Korea Energy Research Institute have found an alternative to vanadium, an important component of redox flow batteries, an active area of research globally.
A study found that vanadium that is mined from the ground can be replaced by readily available molecules made of carbon and oxygen.
Renewable energy technology such as wind and solar can only be adopted on a large scale if a suitable energy storage solution is available. This storage solution can help overcome the intermittent nature of this technology’s power generation and supply power when needed, even if the sun isn’t shining or the wind isn’t blowing.
A reliable energy storage solution should be able to hold a charge for more than eight hours and easily supply it when demand increases. Redox flow batteries are a very preferred solution in this field.
Why are redox flow batteries so good?
Redox flow batteries are electric cells that store energy in an electrolyte instead of an electrode, as in lithium-ion batteries.
Redox flow batteries are better than lithium-ion batteries because they can have a flexible layout, drastically reducing the risk of fire. The power output of the battery can be easily increased by increasing the stack size.
The absence of solid phase changes ensures that the battery has a longer lifespan and delivers superior energy performance even after two decades of operation.
The only drawback of the redox flow battery is that it uses vanadium, which is not a rare earth mineral but is already used in commercial processes and has limited reserves. To avoid a similar fate as lithium, researchers are looking for more abundant substitutes for vanadium, and found them in viologens.
How are viologens useful?
Viologens are organic compounds made from abundantly available elements such as carbon and oxygen that do not need to be mined. Previous research on the use of viologens as a substitute for vanadium faced the obstacle of low solubility in the electrolyte.
The lower solubility of viologens leads to lower battery energy density and instability in charging and discharging processes. To overcome these obstacles, the researchers introduced functional groups into the viologens, which fit into these organic molecules like building blocks and improve their stability.
The research team used sulfonate and ester functional groups, which are water-friendly, making them easier to dissolve in the electrolyte.
The addition of functional groups solved another problem with the use of viologens. Their molecular shape is similar to a sandwich, and sometimes two layers of viologens combine, making them unsuitable for carrying a load.
The researchers added alpha-methyl functional groups that prevent the two viologens from combining, ensuring they are always available for energy storage. The end result of adding these functional groups was that the researchers achieved an energy density twice that of a flow battery made of vanadium.
After 200 charge-discharge cycles, the researchers found that their new battery demonstrated 99.4 percent coulombic efficiency and 92.4 percent capacity retention, both indicators of better performance and stability.
The research findings were published in the journal ACS Applied Materials and Interfaces.
ABOUT THE EDITOR
Ameya Paleja Ameya is a science writer based in Hyderabad, India. A molecular biologist at heart, he traded in the micropipette to write about science during the pandemic, and he doesn’t want to go back. He enjoys writing about genetics, microbes, technology and public policy.